How to Troubleshoot ‘Model is Unstable’ Errors in SOLIDWORKS Simulation

When working in Simulation, there’s a good chance you’ll see one of the following errors at some point.

This indicates there is instability in the setup which will not allow the Simulation solvers to complete the calculations.  Here are some techniques to help you troubleshoot where you may be missing a fixture or contact that causes the instabilities.

SOLIDWORKS Simulation Iterative Solver Stopped

SOLIDWORKS Simulation Model Unstable

Use Soft Spring Option to Allow the Solvers to Run

The Simulation Study Properties has an option “Use soft springs to stabilize the model”.  This applies very weak “springs” to all components in the study to counteract the instability and hopefully allow the solvers to complete.  However if the External Loads are significant, these weak springs will not provide enough constraints.  It’s best to suppress all External Loads and only apply a very small Gravity load.  Gravity will apply to all components which helps determine which one is not properly constrained.

To save yourself a lot of time, suppress all Mesh Controls and create a very coarse Draft-quality mesh.

Depending on how large of study and how much system memory you have available, running with the Intel Direct Sparse solver will provide greater stability.

SOLIDWORKS Simulation Study Properties

You may receive a warning that there are excessive displacements.  Select ‘No’ so it solves with the small displacement method (Large Displacement mode will take much longer).

SOLIDWORKS Simulation Excessive Displacement

Then you can review your Displacement plot and look for the component(s) that has significant movement.  If you don’t see the components on screen, edit the definition of the Displacement plot and set the Deformed Shape scale to Automatic.  Verify that the Fixtures and Contacts are set correctly to ensure they are fully constrained.

SOLIDWORKS Simulation Displacement Plot

SOLIDWORKS Simulation Deformation Scale

Run a Frequency Simulation Study to find Free Bodies

If you run a Frequency study (requires a Simulation Professional or Simulation Premium license), you can pinpoint which component has free body modes, indicating they are not constrained.  Apply the same fixtures as your initial study (you can drag and drop fixtures from a Static study onto a Frequency study), then run the Frequency study for more than 6 modes.  The first six modes would have values close to 0Hz as these are the six free body modes of the unconstrained body.  Be sure to set the Frequency study options to more than 6, as there may be more than one free component.

First six modes will be close to 0, indicating the six free body modes

If the Frequency study fails to run due to numerical difficulties, you can apply a Frequency Shift of slightly more than 0 Hz to help it solve.

Applying a Frequency Shift

Apply Fixed Restraints to Components you Suspect are Moving

As a last resort, if the above two options don’t help, simply apply Fixed Geometry fixtures to a face on any components you think might move.  Then try running the study again.  To save time, be sure to run with a coarse, Draft quality mesh as this is purely for testing.  Then you can narrow down the root source by finding the component(s) that must be Fixed for the study to solve successfully.

Here is a video demonstrating the first two options:

https://youtu.be/deZxHSPerag